longri wrote:At 10,000 feet water boils 18.8°F lower than at sea level but isobutane boils 16.5° lower and propane only 14.7° lower. So your estimate of a 2°F drop per 1000 feet is too optimistic; it should be closer to 1.6°F.
Over a 3,000 meter elevation gain from sea level, the boiling point of hydrocarbons like propane, isobutane, and n-butane is depressed by about 9 degrees Celsius. Converting to Fahrenheit comes out to about 1.6, so I'll buy your number, but the difference between using your number versus the one I used over a 10,000 foot elevation gain is 4 degrees Fahrenheit, an amount which isn't going to have a huge impact on the operation of a stove. In addition, I can work with 2 in my head, and it's easy to remember. It's a rule of thumb not a precise calculation.
longri wrote:It's kind of beside the point since the fuel isn't going to be at ambient temperature. Still air doesn't contain or conduct enough heat. The canister is cooled by fuel vaporization and heated by proximity to the flame, immersion in water and/or warming of the cooking environment (e.g. vestibule). How these factors balance is the key to success in the cold.
Agreed. But you have to understand enough about how canisters work in order to successfully deal with the cold.
I usually tell people that above 50F/10C, you don't really need to worry about mitigating the effects of evaporative cooling as the fuel vaporizes. I tell people that they shouldn't expect an upright canister stove to work well throughout the life of the canister at less than 20 F/-7 C -- and that's using isobutane. These are round numbers. Yes, there are any number of factors that can affect those numbers. The point is that with a few simple numbers, the average person can have some rough rules of thumb that they can remember and use in the field.
To get really precise numbers, you'd have to know the exact temperature, the exact mix of fuel (which changes over time as the propane burns off at a faster rate and to be fully understood requires the calculation of the molar fractions), the exact barometric pressure of the air, the amount of heat loss due to convection, the amount of heat loss due to conduction, and on and on. For the average person, good luck. And out in the field? It's just not practical.
For field usable, practical values:
-Above 50F/10C, don't get too worried about canister cooling for normal boiling of say a liter or less of water.
-Think of 20F/-7C as the lower operating limit of a stove, and then adjust that by about 2 degrees F colder per 1000 feet above sea level, and don't use fuel with n-butane in it. The performance will degrade as one approaches the lower operating limit. Cold weather canister techniques are important as one approaches the lower operating limit (don't use n-butane, warm the canister before use, supply mild heat to the canister during use, insulate from the ground, protect from wind).
If the average person stays within those parameters, they should be fine.
Obviously, one can use can use an upright canister stove as cold as they like so long as they have good cold weather canister techniques. Screw up when it's really cold, and you could be in for a world of hurt. For that reason I generally recommend 20F/-7C as a practical lower limit.